Two-phase frictional pressure drop of R-134a and R-410A refrigerant–oil mixtures in straight tubes and U-type wavy tubes

This study presents single-phase and two-phase pressure drop data for R-134a/oil mixture flowing in a wavy tube with inner diameter of D = 5.07 mm and curvature ratio 2R/D = 5.18 and R-410A/oil mixture flowing in a wavy tube of D = 3.25 mm and 2R/D = 3.91. Both mixtures have oil concentration C = 0%, 1%, 3% and 5% for the tests. The ratio of frictional factor between U-bend in wavy tube and straight tube (fC/fS) is about 3.5 for Re < 2500 and is approximate 2.5 for Re = 3500–25,000 for oil and liquid R-134a mixture flowing in the 5.07 mm diameter wavy tube. The influence of oil concentration on single-phase friction factor is negligible, provided that the properties are based on the mixture of lubricant and refrigerant. The ratio between two-phase pressure gradients of U-bend and straight tube is about 2.5–3.5. This ratio is increased with oil concentration and vapor quality. The influence of oil is augmented at a higher mass flux for liquid spreading around the periphery at an annular flow pattern. Moreover, the influence of lubricant becomes more evident of a U-bend configuration. This is associated the induced swirled flow motion and an early change of flow pattern from stratified to annular flow pattern. The frictional two-phase multiplier for straight tube can be fairly correlated by using the Chisholm correlation for the data having Martinelli parameter X between 0.05 and 1.0. Fridel correlation also shows a good agreement with a mean deviation of 17.6% to all the straight tube data. For the two-phase pressure drop in U-bend, the revised Geary correlation agrees very well with the R-134a and R-410A oil–refrigerant data with a mean deviation of 16.4%.